1. Field of the Invention
The present invention relates in general to disk storage systems and more particularly, to a method and apparatus for providing servo information on a disk in a hard drive assembly.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is typically recorded on concentric tracks on either surface of one or more magnetic recording disks. To facilitate the storage and retrieval of data in an orderly manner, disks are typically organized in blocks called sectors. These sectors are located on the disk by a set of unique specifiers called cylinder (or track), head (or side) and sector number. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads.
The movement of the actuator is controlled by a servo system, utilizing servo information recorded on one or more of the magnetic recording disks. By reading this servo information, the actual radial positions of the heads can be determined, and after comparison with the desired head radial positions, control signals can be sent to move the actuator accordingly. Servo information is typically stored on a disk in one of two ways. In the first, a dedicated servo system, a set of several tracks on the disk or the entire disk surface, is reserved exclusively for storing information associated with the characteristic of the particular drive. Such information includes servo parameters and read/write channel parameters. A servo head reads this information to provide a continuous signal indicating the position of the servo head with respect to the servo disk. In the second type of servo system, the embedded servo system, sectors of servo information are interspersed with sectors of data on each disk surface. As a read head follows the data track around, it regularly reads a fresh sample of servo information from each servo sector with which to control its position.
In the latter case, the servo information is typically written as a plurality of servo bits A, B, C, D in a servo field of each track, as shown in FIG. 1. To duplicate the servo pattern of FIG. 1, the head must first be positioned over a first half of a designated track, and then be directed to write and erase the predetermined sections of the servo field under the direction of a servo controller, to produce the servo pattern as shown in FIG. 1. Upon writing and erasing of the first half of the designated track, the head is microjogged to a position over a second half of the designated track. It is then directed to write and erase the predetermined sections of the servo field.
To write or erase information, a current is provided within the R/W head, which changes the resistance of the R/W head when it is exposed to a magnetic field. Presently, a current of a particular polarity (e.g., positive or negative) is used during the entire erase process, so that the resulting sections of the servo field are erased in only one direction, as shown in FIGS. 2A and 2B. The direct current (D.C.) erasing band resulting from such erasure of each section of the servo field creates a D.C. coupling on the pole of the head during the read operation. This results in a base line shift of the servo signal, and subsequently results in reproducing a servo pattern that appears as shown in FIG. 3. The effects of the D.C. coupling are minor when the track density of a disk is low. However, as track density increases, the D.C. base line shift will result in producing an unstable and unreliable servo signal.
Accordingly, there is a need in the technology for a method and apparatus for providing servo information on a disk which minimizes and/or eliminates the D.C. base line shift during the read operation.